For the original hybrid inflation as well as the supersymmetric F-term and D-term hybrid models , we calculate the level of non-gaussianities and the power spectrum of curvature perturbations generated during the waterfall , taking into account the contribution of entropic modes .
We focus on the regime of mild waterfall , in which inflation continues for more than about 60 e-folds N during the waterfall .
We find that the associated f _ { \mathrm { NL } } parameter goes typically from f _ { \mathrm { NL } } \simeq - 1 / N _ { \mathrm { exit } } in the regime with N \gg 60 , where N _ { \mathrm { exit } } is the number of e-folds between the time of Hubble exit of a pivot scale and the end of inflation , down to f _ { \mathrm { NL } } \sim - 0.3 when N \gtrsim 60 , i.e .
much smaller in magnitude than the current bound from Planck .
Considering only the adiabatic perturbations , the power spectrum is red , with a spectral index n _ { s } = 1 - 4 / N _ { \mathrm { exit } } in the case N \gg 60 , whereas in the case N \gtrsim 60 it increases up to unity .
Including the contribution of entropic modes does not change observable predictions in the first case and the spectral index is too low for this regime to be viable .
In the second case , entropic modes are a relevant source for the power spectrum of curvature perturbations , of which the amplitude increases by several orders of magnitudes .
When spectral index values are consistent with observational constraints , the primordial spectrum amplitude is much larger than the observed value , and can even lead to black hole formation .
We conclude that due to the important contribution of entropic modes , the parameter space leading to a mild waterfall phase is excluded by CMB observations for all the considered models .